1,4-butanediol (BDO) is a versatile chemical intermediate, thanks to its terminal, primary hydroxyl groups and to its chemical resistant nature.
BDO is an important raw material in the synthesis of technically relevant polymers such as thermoplastic urethanes and polyesters, mainly polybutylene terephthalate (PBT), polyester plasticizers, paints, coatings and adhesives.
In the recent years the largest consumer of BDO is tetrahydrofuran (THF), used to produce poly-tetramethylene ether glycol (PTMEG) a key raw material for spandex fibers. Substantial quantities of BDO go into the manufacture of gamma-butyrolactone (GBL) which is in turn used to manufacture N-methyl-pyrrolidone (NMP) and other pyrrolidones.
There are a number of routes to manufacture BDO: the first commercial route was the Reppe process from acetylene and formaldehyde. Other technologies include the production from butadiene, from propylene oxide, from allyl alcohol and more recently through biotransformation process.
Thanks to the availability and low cost of the raw material, in the last twenty years and in the next future the processes based on butane/maleic anhydride tend to dominate the market.
The processes based on butane/maleic anhydride comprise two main groups.
The first group includes all the processes based on the esterification of the maleic anhydride by methanol, ethanol or other alcohols and the following hydrogenation of the dialkyl ester, usually as dimethyl maleate (DMM), to butanediol and/or, depending on the conditions of pressure and temperature and on the catalyst, to tetrahydrofuran and gamma-butyrolactone.
The second group on the contrary includes the processes based on the direct hydrogenation of the maleic anhydride or maleic acid to BDO and/or THF and GBL. Even if theoretically these processes appear to be simpler, with fewer processing steps (both the maleic anhydride purification and the esterification are not necessary), in the industrial application they met with poor success, probably due to the high cost of the catalyst and of the sophisticated materials of construction necessary in presence of maleic acid at high temperature.
Considering again the processes of the first group based on the esterification of the maleic anhydride, another distinction can be made between the processes based on the hydrogenation in vapor phase and the processes based on the hydrogenation in mixed liquid/vapor phase.
The new process introduced in the present invention belongs to the hydrogenation processes of diesters of the maleic anhydride in mixed phase.
Irrespective of the vapor or mixed phase, it is believed that the reaction of dimethyl maleate to 1,4-butanediol proceeds through at least two intermediates, as follows:

In conclusion, 1,4-butanediol, tetrahydrofuran and gamma-butyrolactone are products of reaction which are converted one to another by addition/subtraction of hydrogen and/or water. The products distribution may be changed by adjusting the operating parameters of the hydrogenolysis reaction or the type of catalysts.
In the prior art several inventions have been applied on the hydrogenation of maleic anhydride esters to produce 1,4-butanediol.
U.S. Pat. No. 2,110,488 can be considered the first application concerning the hydrogenation of an ester of aliphatic carboxylic acids to produce aliphatic alcohols using a catalyst consisting of copper oxide and chromium oxide, within a temperature range from 250 to 300° C. and within a pressure range from 150 to 300 atmospheres.
On the other hand U.S. Pat. No. 4,032,458 is the first application on the production of 1,4-butanediol, where a solution containing maleic acid is first subject to esterification by a monohydric alcohol and the dialkyl ester of maleic acid is hydrogenated in a two reaction steps, the first step to saturate the double bond present in the maleic acid and the second step to produce 1,4-butanediol and the monohydric alcohol. Both reaction steps uses a copper-chromite catalyst. Reaction temperatures are in the range from 100 to 300° C., reaction pressures from 172 to 241 barg, being the catalyst dissolved in slurry.
U.S. Pat. No. 4,172,961 is another application of copper chromite catalyst in slurry, to hydrogenate dibutyl butoxy succinate to 1,4-butanediol.
WO patent 82/03854 is a generic process to hydrogenate a carboxylic acid ester in vapor phase with a catalyst comprising a mixture of copper oxide and zinc oxide. One of the several possible applications concerns the production of 1,4-butanediol from an ester of maleic, fumaric or succinic acid.
U.S. Pat. No. 4,584,419 consists in a process to hydrogenate an ester of maleic anhydride to butanediol in vapor phase in the presence of a copper chromite catalyst.
U.S. Pat. No. 4,656,297 is another process to produce butanediol by the hydrogenation in vapor phase of dimethylsuccinate with copper chromite catalyst, by adding methanol to the ester feed.
U.S. Pat. No. 4,751,334 is also an hydrogenation process in vapor phase with Cu—Cr or Cu—Zn oxide catalyst, to produce 1,4-butanediol from diethyl maleate in two or three adiabatic hydrogenolysis zones in series.
U.S. Pat. No. 5,872,276 refers to a process to hydrogenate dialkyl maleate to dialkyl succinate in liquid phase at a pressure of from 50 to 400 bar, at temperature of from 30 to 160° C. and at hydrogen/feed molar ratio from 20 to 40. The catalyst consists of pressed powders of the elements of the iron subgroup with addition of elements of transition groups IV or V, having particular compressive strength and internal surface area.
WO patent 99/52845 represents a process for producing BDO by vapor phase hydrogenation of different types of feed, including dimethyl maleate, characterized by the addition of carbon monoxide to the reaction.
U.S. Pat. Nos. 6,100,410, 6,239,292, 6,274,743, 6,350,924 and 6,433,192 describe different special integrations between a maleic anhydride plant and a BDO production plant, wherein the maleic anhydride absorbed in an organic solvent is first esterified and the resulting maleic ester is stripped by a stream of hydrogen and then subject to an hydrogenation in vapor phase.
U.S. Pat. Nos. 6,137,016 and 2007/0260073 provide a process for the purification of BDO containing minor amount of cyclic acetal, by hydrogenating the butanediol in presence of minor amount of water and under conditions of temperature and pressure typical of the liquid/vapor reactions.
U.S. Pat. No. 6,191,322 is a process for the production of BDO by mixed phase catalytic hydrogenation of gamma-butyrolactone or succinic anhydride esters or their mixtures in two catalytic steps in series with injection of cold hydrogen between the reaction stages, at pressure of from 75 to 90 bar and temperature between 160 and 250° C. The catalyst can be selected between copper and zinc oxide or copper chromite.
U.S. Pat. No. 6,248,906 is a process where the maleic anhydride ester is subject to two subsequent hydrogenation steps, where the primary hydrogenation produces mainly GBL and THF and the secondary hydrogenation converts a fraction of GBL to BDO.
U.S. Pat. No. 6,288,245 teaches a process to convert the BDO produced by vapor phase hydrogenation of maleic ester to THF through a second stage reactor containing silica-alumina catalyst.
U.S. Pat. No. 6,433,193 refers to an integration between maleic anhydride and butanediol productions, where the maleic anhydride is recovered by absorption in gamma-butyrolactone and the resulting mixture of maleic anhydride and GBL is subjected to an hydrogenation step under conditions favoring the formation of THF and GBL.
U.S. Pat. No. 6,620,949 is another process of integration where the maleic anhydride used to produce BDO, GBL and THF is produced by partial condensation of the vapor effluent from a maleic anhydride catalytic oxidation reactor.
U.S. Pat. No. 6,844,452 is a hydrogenation process in vapor phase for production of BDO and THF from dimethyl or diethyl maleate, characterized by three hydrogenation zones in series using different catalysts, such arrangement permitting to accept minor amounts of acidic material in the feed, to enhance the yields of THF and to reduce the formation of cyclic acetals.
U.S. Pat. No. 6,936,727 refers to a particular scheme of vapor phase hydrogenation, using catalyst and conditions favorable to the formation of tetrahydrofuran, characterized by the liquid feed which is divided in two parts fed separately to two subsequent vaporizer/reactor systems, allowing in this way an overall lower circulation of hydrogen to maintain the reactor feed above its dew point temperature.
CN patent 101307042 relates to a method for producing BDO together with THF and GBL, by using two catalyst bed layers, the first containing copper oxide and copper chromite or zinc oxide with one among Ba, Mg, Ti, Ce, Si, Zr and Mn oxide additive, the second containing copper oxide, alumina and one of the above described additive.
The aim of the U.S. Pat. No. 7,598,404 is the same of the above mentioned U.S. Pat. No. 6,936,727: in a process targeted to producing mainly tetrahydrofuran, the feed is first hydrogenated in a pre-reactor zone comprising catalyst which favors the carbon double bond saturation, i.e. the production of dimethyl succinate from dimethyl maleate. In this way the heat of reaction may be utilized in the evaporation of some of the liquid feed. Accordingly to the patent, in the second reaction zone, where THF and optionally diol and/or lactone are produced, the reaction occurs strictly under conditions of complete vaporization.
CN patent 101747149 discloses a method for preparing BDO by two stages hydrogenation in series by using maleic acid dialkyl ester, where the effluent from the first reaction step is cooled and separated in two phases and the resulting liquid product is contacted again with hydrogen in the second hydrogenation section.
Most of the above mentioned and other patents on the production of butanediol by hydrogenation of a dialkyl maleate, refers to a reaction in vapor phase. The operation in vapor phase at high pressure needs a large amount of recirculating hydrogen rich gas.
Moreover the conversion of DMM in a single reactor, where both the hydrogenation (double bond saturation) and the hydrogenolysis reactions occur at the same time, being the hydrogenation reaction the more exothermic step and also the quicker as rate of reaction, causes localized high temperatures (hot spot) in the first part of the catalytic bed difficult to control and producing unwelcome by-products, butanol and other, and risk of catalyst decay.
The already mentioned U.S. Pat. No. 7,598,404 refers at page 3: In conventional vapour phase reactions with hydrogen the capital and operating costs, particularly energy and other utilities requirements, are largely determined by the flow rate of the gas feed to the vaporiser . . . . The size of the compressors, heat exchangers and interconnecting pipework is dictated by the cycle gas flow rate as is the power required for compression and the heat required to be added to the reactor feed and removed from the reactor product. There is therefore a strong incentive to minimize the cycle gas flow rate within a particular process.
It is clear the reduction of the cycle gas below a certain limit, causes the reaction to be any more in vapor phase and to enter in the mixed liquid/vapor conditions. This operation with a reduced gas circulation, even if of course it is advantageous both as capital and operating cost, owing to the high exothermic effect of the hydrogenation reaction, results in a more difficult control of the reaction temperature. In fact, as already mentioned, excessive temperature of the reaction involves both an higher formation of undesired by-products and a reduced life of the catalyst. The by-products are certainly cause of reduced efficiency (higher consumption of raw materials and utilities) and, in some cases, are detrimental for the quality of the product.
The above mentioned U.S. Pat. No. 6,191,322 is one of the very few patent for the production of BDO in mixed phase. The patent solves the problem connected to the temperature control, first by using as feedstock an already partially hydrogenated product, i.e. gamma-butyrolactone or dimethyl succinate or a mixture of both compounds, second by using a multistage reactor and by cooling down the mixture between the reaction stages by injecting cold hydrogen.
It is clear that the main limitation of this patent is related to the type of feedstock, which needs a further separate hydrogenation plant to transform dimethylmaleate into dimethylsuccinate or gamma-butyrolactone.
It is object of the present invention a method to produce 1,4-butanediol from dialkyl maleates in a single hydrogenation plant, by avoiding the expensive process in vapor phase.